1. Field of the Invention
The present invention relates generally to systems for controlling stepping motors and, more particularly, to systems for controlling the operation of a dc stepping motor utilized for positioning a read/write head on a magnetic data storage disk.
2. Description of the Prior Art
In the field of electronic computer technology, it is typical to store data in binary form on the face of a rotatable disk, which face is coated wth a magnetizable substance. In operation, such disks are spun like phonograph records and the binary data is encoded upon, or retrieved from, the face of the disk by a movable magnetic transducer device called a read/write head. More particularly, the binary information is magnetically encoded on the face of the disk in concentric rings, called tracks, and the read/write head is selectively positionable in a direction radial of the disk to select a particular track at which the information is to be recorded or retrieved.
On such magnetic recording disks, it is known to have track densities ranging from about forty tracks per inch of the radius, for so-called floppy disks, to densities exceeding one thousand tracks per inch of the radius of the disk, for so-called hard disks. Accordingly, precise positioning of the read/write head is necessary so that the head can accurately access a particular desired track on the surface of the disk.
Not only is it important that the desired track on a magnetic recording disk be located accurately, but it is also important that the locating operation be conducted in as short a time as possible. The time period required to gain access to a particular desired track on a magnetic recording disk by positioning the read/write head over that track is usually referred to as the seek time. Minimization of seek time is desirable because, during the period that a particular track is being sought and data on that track is being located, the central processing unit of the computer is unable to proceed with the calculation, or other processing, for which the sought data is required input. For most magnetic disk memories, typical seek times range between ten and one hundred milliseconds.
In conventional magnetic recording disk systems, the read/write head is driven to seek a desired track on the disk by means of a dc (direct current) motor called a stepping motor. Such motors translate electrical pulses, which may be understood to be commands from the controller for the disk system, to rotational mechanical movement. In operation, the output shaft of the motor rotates through a specific angular rotation for each electrical command pulse. Such incremental rotational movement is converted to linear movement of the read/write head by a so-called capstan mechanism or the like. In typical systems, it is conventional to control the stepping motors in incremental steps of 1.8 degrees each or in incremental steps of 0.9 degrees each.
One requirement of operation of such stepping motors relates to the repeatability of the precision of the angular positioning of the output shaft of the motor at the incremental steps. For example, if a required angular stop is at 270.degree., it is highly desirable that such a stop be precisely repeatable and that the same stop position be reached regardless of whether the direction of rotation of the shaft of the motor preceding the stop was clockwise or counterclockwise.
A typical problem with stepping motors utilized to position a read/write head on a magnetic storage disk is that the output shafts of such motors may cease rotation by a fraction of a degree or more prior to reaching the desired angular position. This is called "stopping short". Because the stepping motor is required to rotate both in the clockwise and counterclockwise direction, to correspondingly extend or retract the read/write head across the face of the magnetic recording disk, the stopping-short phenomenon will cause inherent inaccuracies in positioning of the read/write head. That is, for a desired angular position, the actual position at which the motor stops will depend upon whether the motor approaches the desired stop position through clockwise or counterclockwise rotation. In practice, to compensate for the stopping-short problem, it is conventional to control a stepping motor so that its final approach to the desired stop position is always in the same direction, say in the counterclockwise direction. This is accomplished by requiring the motor to always rotate one stop beyond the desired stop and then returns to the desired stop. So, if the motor in the example is initially rotating clockwise, it will continue one stop beyond the desired 270.degree. stop, and then made one incremental counterclockwise return step toward the 270.degree. stop. Or, if the motor in the example is initially rotating counterclockwise, it will rotate one incremental stop or so beyond the desired 270.degree. stop, then rotate clockwise until it goes one incremental stop beyond the 270.degree. stop, and then reverse direction a third time to move counterclockwise toward the desired 270.degree. stop.
The above-described procedure can be understood to compensate for the stopping-short problem by requiring certain mechanical movements prior to each stop. Thus, the procedure might be called "stepping too far and coming back." An inherent difficulty with this procedure is that the additional mechanical movements are time consuming and, therefore, contribute to relatively long seek times.
There may be both mechanical and electrical causes for the above-described stopping-short problem. For instance, friction in the mechanical elements of the read/write head positioning system may cause a stepping motor to stop short. Most often, however, magnetic hysteresis in the ferromagnetic cores of the stepping motor causes the stopping short problem, due to the fact that the strength of the magnetic field exerted by the core member depends upon whether the electric current passing through the windings surrounding the core is inncreasing or decreasing, and not just upon the amount of the current. In any case, the above-discussed compensation technique introduces time consuming mechanical movements.